Magtheridon's Lair is the raid wing of the Hellfire Citadel dungeon in Outland where you fight the brutish pit lordMagtheridon himself. You can find the entrance to Magtheridon's Lair on the very bottom of the western side of the Citadel (opposite side from the other 3 entrances). It is a quick raid encounter (like Onyxia's Lair, which contains four "guard mobs" before the boss encounter, Onyxia).